From Wikipedia, the free encyclopedia
Ball and stick model of zwitterionic citrulline
IUPAC name
2-Amino-5-(carbamoylamino)pentanoic acid[1]
3D model (JSmol)
1725417, 1725415 D, 1725416 L
ECHA InfoCard 100.006.145 Edit this at Wikidata
EC Number
  • 211-012-2
774677 L
MeSH Citrulline
  • InChI=1S/C6H13N3O3/c7-4(5(10)11)2-1-3-9-6(8)12/h4H,1-3,7H2,(H,10,11)(H3,8,9,12) ☒N
  • NC(CCCNC(N)=O)C(O)=O
Molar mass 175.188 g·mol−1
Appearance White crystals
Odor Odourless
log P −1.373
Acidity (pKa) 2.508
Basicity (pKb) 11.489
232.80 J K−1 mol−1
254.4 J K−1 mol−1
Related compounds
Related alkanoic acids
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

The organic compound citrulline is an α-amino acid.[2] Its name is derived from citrullus, the Latin word for watermelon. Although named and described by gastroenterologists since the late 19th century, it was first isolated from watermelon in 1914 by Japanese researchers Yotaro Koga and Ryo Odake[3][note 1] and further codified by Mitsunori Wada of Tokyo Imperial University in 1930.[4] It has the formula H2NC(O)NH(CH2)3CH(NH2)CO2H. It is a key intermediate in the urea cycle, the pathway by which mammals excrete ammonia by converting it into urea. Citrulline is also produced as a byproduct of the enzymatic production of nitric oxide from the amino acid arginine, catalyzed by nitric oxide synthase.[5]


Citrulline can be derived from:

Citrulline is made from ornithine and carbamoyl phosphate in one of the central reactions in the urea cycle. It is also produced from arginine as a byproduct of the reaction catalyzed by NOS family (NOS; EC[6] It is also prevalent in trichohyalin at the inner root sheath and medulla of hair follicles, where it is synthesized from arginine.[7] Arginine is first oxidized into N-hydroxyl-arginine, which is then further oxidized to citrulline concomitant with release of nitric oxide.

Citrulline is also made by enterocytes of the small intestine.[2][8]


Citrulline is a metabolic intermediate within the urea cycle, which is the pathway by which mammals excrete ammonia by converting it into urea. Citrulline is also produced as a byproduct of the enzymatic production of nitric oxide from the amino acid arginine, catalyzed by nitric oxide synthase. In the yeast species Saccharomyces cerevisiae, citrulline is a metabolic intermediate in the latter, cytosolic half of the arginine biosynthesis pathway.[9]

Several proteins contain citrulline as a result of a post-translational modification. These citrulline residues are generated by a family of enzymes called peptidylarginine deiminases (PADs), which convert arginine into citrulline in a process called citrullination or deimination with the help of calcium ions. Proteins that normally contain citrulline residues include myelin basic protein (MBP), filaggrin, and several histone proteins, whereas other proteins, such as fibrin and vimentin are susceptible to citrullination during cell death and tissue inflammation.

Circulating citrulline concentration is a biomarker of intestinal functionality.[10][11]

See also[edit]


  1. ^ Early references spell Ryo Odake's name as Ryo Othake.


  1. ^ "Citrulline - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification. Retrieved 1 May 2012.
  2. ^ a b Banerjee, Aryamitra (2014-01-01), Gupta, Ramesh C. (ed.), "Chapter 15 - Gastrointestinal toxicity biomarkers", Biomarkers in Toxicology, Boston: Academic Press, pp. 269–277, doi:10.1016/b978-0-12-404630-6.00015-4, ISBN 978-0-12-404630-6, S2CID 88798984, retrieved 2020-11-10
  3. ^ Fragkos, Konstantinos C.; Forbes, Alastair (September 2011). "Was citrulline first a laxative substance? The truth about modern citrulline and its isolation" (PDF). Nihon Ishigaku Zasshi. [Journal of Japanese History of Medicine]. 57 (3): 275–292. ISSN 0549-3323. PMID 22397107. Archived (PDF) from the original on 2022-10-09.
  4. ^ Fearon, William Robert (1939). "The Carbamido Diacetyl Reaction: A Test For Citrulline". Biochemical Journal. 33 (6): 902–907. doi:10.1042/bj0330902. PMC 1264464. PMID 16746990.
  5. ^ "Nos2 - Nitric Oxide Synthase". Uniprot Consortium. Retrieved 10 February 2015.
  6. ^ Cox M, Lehninger AL, Nelson DR (2000). Lehninger principles of biochemistry (3rd ed.). New York: Worth Publishers. p. 449. ISBN 978-1-57259-153-0. Retrieved 13 March 2020.
  7. ^ Rogers, G. E.; Rothnagel, J. A. (1983). "A sensitive assay for the enzyme activity in hair follicles and epidermis that catalyses the peptidyl-arginine-citrulline post-translational modification". Current Problems in Dermatology. 11: 171–184. doi:10.1159/000408673. ISBN 978-3-8055-3752-0. PMID 6653155.
  8. ^ DeLegge, Mark H. (2019-01-01), Corrigan, Mandy L.; Roberts, Kristen; Steiger, Ezra (eds.), "Chapter 7 - Enteral Access and Enteral Nutrition in Patients With Short Bowel Syndrome", Adult Short Bowel Syndrome, Academic Press, pp. 81–96, doi:10.1016/b978-0-12-814330-8.00007-x, ISBN 978-0-12-814330-8, S2CID 81295089, retrieved 2020-11-10
  9. ^ "Saccharomyces cerevisiae citrulline biosynthesis".
  10. ^ Fragkos, Konstantinos C.; Forbes, Alastair (2017-10-12). "Citrulline as a marker of intestinal function and absorption in clinical settings: A systematic review and meta-analysis". United European Gastroenterology Journal. 6 (2): 181–191. doi:10.1177/2050640617737632. PMC 5833233. PMID 29511548.
  11. ^ Crenn, P.; et al. (2000). "Post-absorptive plasma citrulline concentration is a marker of intestinal failure in short bowel syndrome patients". Gastroenterology. 119 (6): 1496–505. doi:10.1053/gast.2000.20227. PMID 11113071.